There has been a failure in the field of data transmission systems to recognize the potential of a television signal, and in particular, the large bandwidth available using a television signal. At present, methods of encoding data into a television signal have been restricted to relatively low bandwidth and inefficient use of the television signal medium. The reason for this appears to be that the data encoding methods in use were designed more than twenty-five years ago, and the bandwidth required for the original purpose is relatively low. For instance, these methods were designed to allow a television signal to carry closed-captioning and/or teletext information. The data rate needed for carrying this information is extremely low, and though the bit rate of the data transmission was fairly high (e.g., 4.5 million bits per second in bursts), the overall bandwidth utilization of the television signal generally does not exceed 45,000 bits per second, Moreover, the data encoding methods used for captioning and teletext are fairly rudimentary. In this regard, these methods divide a signal line of a television picture up into a number of time slices, and then impress digital information into the slots as a series of black or white spots, representing the digital values of zero and one. The data bytes are encoded into changing voltages (i.e., black and white spots), and decoded back into bytes by using a commonly available electronic device, such as UART (Universal Asynchronous Receiver Transmitter) or an ACIA (Asynchronous Communications Interface Adapter). One limitation of this "binary coding" scheme is that it limits the data transmission rate. In order to achieve greater transmission rates, data has been encoded into complex waveforms, such as tones, which are then phase, frequency, and amplitude modulated in order to carry the information. For instance, this method is used in data modems for use in longer distance high-speed communications. Other forms of encoding for high-speed data transmission include RF (Radio Frequency) modulation, networks such as Ethernet or Arcnet, QAM (Quadrature Amplitude Modulation), ASK (Amplitude Shift Keying), PSK (Phase Shift Keying), FSK (Frequency Shift Keying), TCM (Trellis Coded Modulation) and QPSK (Quadrature Phase Shift Keying). All of the foregoing methods encode data on to waveforms for transmission, which is the next step beyond raw digital data.
In order to derive further benefits from the use of the television signal as a transmission medium, it would be advantageous to encode multiple data bits in parallel as discreet levels into the television image.
The "information highway" of the future relies on high-speed data transmission to distribute image, sound and video to multiple access points worldwide. Currently, the distribution bottleneck is the data rate (defined in bits per second) capabilities of the communications medium in use. For example, one minute of compressed digital video data may be represented by approximately ten megabytes of data. Currently available communications mediums include voice grade modems, leased line modems, ISDN services, fiber-optic or high-speed land-line links, radio modems and satellite data links.
Prior art approaches to achieving higher data transmission rates have typically involved trying to compress the data or to pack more bits per basic transmission unit (baud) within the same channel bandwidth. For example, a 9.6 Kbps modem for use on standard two-wire telephone lines operates within the 3 Khz bandwidth available. To reach 9.6 Kbps, it may use a basic baud rate of 2400 baud, and encode 4 bits per baud. The "baud" defines a number of transitions made on the base carrier each second, and the number of bits per baud multiplied by the baud rate provides a number of bits per second (bps).
Another aspect of the present invention is directed to the encoding of digital data. Prior art data encoding methods for encoding digital information have used two voltage levels, where each voltage level represents a single bit. In this respect, a first voltage level represents a digital value "0," while a second voltage level represents a digital value "1." As a result, a set of eight of these voltage levels is needed to encode one byte of digital data. Data bytes are encoded into changing voltages and decoded back into bytes. This is typically done by using a UART or an ACIA. UARTs convert parallel data (usually eight-bit words) to a serial data stream for transmission over a single wire cable and likewise convert a received serial bit stream to parallel words. The serial data stream is comprised of a signal having two voltage levels, one representing a digital "0," the other representing a digital "1."
In many cases, the data rate achievable by UARTs and ACIAs is insufficient for the desired application. In order to achieve greater data rates for high-speed communications, data has been encoded into complex waveforms such as tones, which are then phase, frequency and amplitude modulated. As mentioned above, data has been encoded using RF, QAM, ASK, PSK, FSK, TCM and QPSK. All of the foregoing methods encode data into AC waveforms for transmission.
The present invention provides a novel system for transmitting and receiving digital data using a conventional television signal for data transfer. In a preferred embodiment of the present invention an encoding system is used which overcomes the data transfer rate limitations of the prior art encoding systems, and provides a system for encoding multiple data bits in parallel as transitions between discrete levels.